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<div class="titlepage"><div><div><h3 class="title">
<a name="asio.reference.asynchronous_operations"></a><a class="link" href="asynchronous_operations.html" title="Requirements on asynchronous operations">Requirements
      on asynchronous operations</a>
</h3></div></div></div>
<p>
        This section uses the names <code class="computeroutput">Alloc1</code>, <code class="computeroutput">Alloc2</code>, <code class="computeroutput">alloc1</code>,
        <code class="computeroutput">alloc2</code>, <code class="computeroutput">Args</code>, <code class="computeroutput">CompletionHandler</code>,
        <code class="computeroutput">completion_handler</code>, <code class="computeroutput">Executor1</code>, <code class="computeroutput">Executor2</code>,
        <code class="computeroutput">ex1</code>, <code class="computeroutput">ex2</code>, <code class="computeroutput">f</code>, <code class="literal"><span class="emphasis"><em>i</em></span></code>,
        <code class="literal"><span class="emphasis"><em>N</em></span></code>, <code class="computeroutput">Signature</code>, <code class="computeroutput">token</code>,
        <code class="literal">T[sub <span class="emphasis"><em>i</em></span></code>], <code class="literal">t[sub <span class="emphasis"><em>i</em></span></code>],
        <code class="computeroutput">work1</code>, and <code class="computeroutput">work2</code> as placeholders for specifying
        the requirements below.
      </p>
<h5>
<a name="asio.reference.asynchronous_operations.h0"></a>
        <span><a name="asio.reference.asynchronous_operations.general_asynchronous_operation_concepts"></a></span><a class="link" href="asynchronous_operations.html#asio.reference.asynchronous_operations.general_asynchronous_operation_concepts">General
        asynchronous operation concepts</a>
      </h5>
<p>
        An <span class="emphasis"><em>initiating function</em></span> is a function which may be called
        to start an asynchronous operation. A <span class="emphasis"><em>completion handler</em></span>
        is a function object that will be invoked, at most once, with the result
        of the asynchronous operation.
      </p>
<p>
        The lifecycle of an asynchronous operation is comprised of the following
        events and phases:
      </p>
<p>
        &#8212; Event 1: The asynchronous operation is started by a call to the initiating
        function.
      </p>
<p>
        &#8212; Phase 1: The asynchronous operation is now <span class="emphasis"><em>outstanding</em></span>.
      </p>
<p>
        &#8212; Event 2: The externally observable side effects of the asynchronous operation,
        if any, are fully established. The completion handler is submitted to an
        executor.
      </p>
<p>
        &#8212; Phase 2: The asynchronous operation is now <span class="emphasis"><em>completed</em></span>.
      </p>
<p>
        &#8212; Event 3: The completion handler is called with the result of the asynchronous
        operation.
      </p>
<p>
        In this library, all functions with the prefix <code class="computeroutput">async_</code> are initiating
        functions.
      </p>
<h5>
<a name="asio.reference.asynchronous_operations.h1"></a>
        <span><a name="asio.reference.asynchronous_operations.completion_tokens_and_handlers"></a></span><a class="link" href="asynchronous_operations.html#asio.reference.asynchronous_operations.completion_tokens_and_handlers">Completion
        tokens and handlers</a>
      </h5>
<p>
        Initiating functions:
      </p>
<p>
        &#8212; are function templates with template parameter <code class="computeroutput">CompletionToken</code>;
      </p>
<p>
        &#8212; accept, as the final parameter, a <span class="emphasis"><em>completion token</em></span> object
        <code class="computeroutput">token</code> of type <code class="computeroutput">CompletionToken</code>;
      </p>
<p>
        &#8212; specify a <span class="emphasis"><em>completion signature</em></span>, which is a call signature
        (C++Std [func.def]) <code class="computeroutput">Signature</code> that determines the arguments
        to the completion handler.
      </p>
<p>
        An initiating function determines the type <code class="computeroutput">CompletionHandler</code>
        of its completion handler function object by performing <code class="computeroutput">typename async_result&lt;decay_t&lt;CompletionToken&gt;,
        Signature&gt;::completion_handler_type</code>. The completion handler object
        <code class="computeroutput">completion_handler</code> is initialized with <code class="computeroutput">forward&lt;CompletionToken&gt;(token)</code>.
        [<span class="emphasis"><em>Note:</em></span> No other requirements are placed on the type
        <code class="computeroutput">CompletionToken</code>. &#8212;<span class="emphasis"><em>end note</em></span>]
      </p>
<p>
        The type <code class="computeroutput">CompletionHandler</code> must satisfy the requirements of
        <code class="computeroutput">Destructible</code> (C++Std [destructible]) and <code class="computeroutput">MoveConstructible</code>
        (C++Std [moveconstructible]), and be callable with the specified call signature.
      </p>
<p>
        In this library, all initiating functions specify a <span class="emphasis"><em>Completion
        signature</em></span> element that defines the call signature <code class="computeroutput">Signature</code>.
        The <span class="emphasis"><em>Completion signature</em></span> elements in this Technical
        Specification have named parameters, and the results of an asynchronous operation
        are specified in terms of these names.
      </p>
<h5>
<a name="asio.reference.asynchronous_operations.h2"></a>
        <span><a name="asio.reference.asynchronous_operations.automatic_deduction_of_initiating_function_return_type"></a></span><a class="link" href="asynchronous_operations.html#asio.reference.asynchronous_operations.automatic_deduction_of_initiating_function_return_type">Automatic
        deduction of initiating function return type</a>
      </h5>
<p>
        The return type of an initiating function is <code class="computeroutput">typename async_result&lt;decay_t&lt;CompletionToken&gt;,
        Signature&gt;::return_type</code>.
      </p>
<p>
        For the sake of exposition, this library sometimes annotates functions with
        a return type <span class="emphasis"><em><code class="literal">DEDUCED</code></em></span>. For every
        function declaration that returns <span class="emphasis"><em><code class="literal">DEDUCED</code></em></span>,
        the meaning is equivalent to specifying the return type as <code class="computeroutput">typename
        async_result&lt;decay_t&lt;CompletionToken&gt;, Signature&gt;::return_type</code>.
      </p>
<h5>
<a name="asio.reference.asynchronous_operations.h3"></a>
        <span><a name="asio.reference.asynchronous_operations.production_of_initiating_function_return_value"></a></span><a class="link" href="asynchronous_operations.html#asio.reference.asynchronous_operations.production_of_initiating_function_return_value">Production
        of initiating function return value</a>
      </h5>
<p>
        An initiating function produces its return type as follows:
      </p>
<p>
        &#8212; constructing an object <code class="computeroutput">result</code> of type <code class="computeroutput">async_result&lt;decay_t&lt;CompletionToken&gt;,
        Signature&gt;</code>, initialized as <code class="computeroutput">result(completion_handler)</code>;
        and
      </p>
<p>
        &#8212; using <code class="computeroutput">result.get()</code> as the operand of the return statement.
      </p>
<p>
        [<span class="emphasis"><em>Example:</em></span> Given an asynchronous operation with <span class="emphasis"><em>Completion
        signature</em></span> <code class="computeroutput">void(R1 r1, R2 r2)</code>, an initiating function
        meeting these requirements may be implemented as follows:
      </p>
<pre class="programlisting">template&lt;class CompletionToken&gt;
auto async_xyz(T1 t1, T2 t2, CompletionToken&amp;&amp; token)
{
  typename async_result&lt;decay_t&lt;CompletionToken&gt;, void(R1, R2)&gt;::completion_handler_type
    completion_handler(forward&lt;CompletionToken&gt;(token));

  async_result&lt;decay_t&lt;CompletionToken&gt;, void(R1, R2)&gt; result(completion_handler);

  // initiate the operation and cause completion_handler to be invoked with
  // the result

  return result.get();
}
</pre>
<p>
        For convenience, initiating functions may be implemented using the <code class="computeroutput">async_completion</code>
        template:
      </p>
<pre class="programlisting">template&lt;class CompletionToken&gt;
auto async_xyz(T1 t1, T2 t2, CompletionToken&amp;&amp; token)
{
  async_completion&lt;CompletionToken, void(R1, R2)&gt; init(token);

  // initiate the operation and cause init.completion_handler to be invoked
  // with the result

  return init.result.get();
}
</pre>
<p>
        &#8212;<span class="emphasis"><em>end example</em></span>]
      </p>
<h5>
<a name="asio.reference.asynchronous_operations.h4"></a>
        <span><a name="asio.reference.asynchronous_operations.lifetime_of_initiating_function_arguments"></a></span><a class="link" href="asynchronous_operations.html#asio.reference.asynchronous_operations.lifetime_of_initiating_function_arguments">Lifetime
        of initiating function arguments</a>
      </h5>
<p>
        Unless otherwise specified, the lifetime of arguments to initiating functions
        shall be treated as follows:
      </p>
<p>
        &#8212; If the parameter has a pointer type or has a type of lvalue reference to
        non-const, the implementation may assume the validity of the pointee or referent,
        respectively, until the completion handler is invoked. [<span class="emphasis"><em>Note:</em></span>
        In other words, the program must guarantee the validity of the argument until
        the completion handler is invoked. &#8212;<span class="emphasis"><em>end note</em></span>]
      </p>
<p>
        &#8212; Otherwise, the implementation must not assume the validity of the argument
        after the initiating function completes. [<span class="emphasis"><em>Note:</em></span> In other
        words, the program is not required to guarantee the validity of the argument
        after the initiating function completes. &#8212;<span class="emphasis"><em>end note</em></span>] The
        implementation may make copies of the argument, and all copies shall be destroyed
        no later than immediately after invocation of the completion handler.
      </p>
<h5>
<a name="asio.reference.asynchronous_operations.h5"></a>
        <span><a name="asio.reference.asynchronous_operations.non_blocking_requirements_on_initiating_functions"></a></span><a class="link" href="asynchronous_operations.html#asio.reference.asynchronous_operations.non_blocking_requirements_on_initiating_functions">Non-blocking
        requirements on initiating functions</a>
      </h5>
<p>
        An initiating function shall not block (C++Std [defns.block]) the calling
        thread pending completion of the outstanding operation.
      </p>
<p>
        [std_note Initiating functions may still block the calling thread for other
        reasons. For example, an initiating function may lock a mutex in order to
        synchronize access to shared data.]
      </p>
<h5>
<a name="asio.reference.asynchronous_operations.h6"></a>
        <span><a name="asio.reference.asynchronous_operations.associated_executor"></a></span><a class="link" href="asynchronous_operations.html#asio.reference.asynchronous_operations.associated_executor">Associated
        executor</a>
      </h5>
<p>
        Certain objects that participate in asynchronous operations have an <span class="emphasis"><em>associated
        executor</em></span>. These are obtained as specified in the sections below.
      </p>
<h5>
<a name="asio.reference.asynchronous_operations.h7"></a>
        <span><a name="asio.reference.asynchronous_operations.associated_i_o_executor"></a></span><a class="link" href="asynchronous_operations.html#asio.reference.asynchronous_operations.associated_i_o_executor">Associated
        I/O executor</a>
      </h5>
<p>
        An asynchronous operation has an associated executor satisfying the <a class="link" href="Executor1.html" title="Executor requirements"><code class="computeroutput">Executor</code></a> requirements.
        If not otherwise specified by the asynchronous operation, this associated
        executor is an object of type <code class="computeroutput">system_executor</code>.
      </p>
<p>
        All asynchronous operations in this library have an associated executor object
        that is determined as follows:
      </p>
<p>
        &#8212; If the initiating function is a member function, the associated executor
        is that returned by the <code class="computeroutput">get_executor</code> member function on the
        same object.
      </p>
<p>
        &#8212; If the initiating function is not a member function, the associated executor
        is that returned by the <code class="computeroutput">get_executor</code> member function of the
        first argument to the initiating function.
      </p>
<p>
        Let <code class="computeroutput">Executor1</code> be the type of the associated executor. Let <code class="computeroutput">ex1</code>
        be a value of type <code class="computeroutput">Executor1</code>, representing the associated executor
        object obtained as described above.
      </p>
<h5>
<a name="asio.reference.asynchronous_operations.h8"></a>
        <span><a name="asio.reference.asynchronous_operations.associated_completion_handler_executor"></a></span><a class="link" href="asynchronous_operations.html#asio.reference.asynchronous_operations.associated_completion_handler_executor">Associated
        completion handler executor</a>
      </h5>
<p>
        A completion handler object of type <code class="computeroutput">CompletionHandler</code> has an
        associated executor of type <code class="computeroutput">Executor2</code> satisfying the <a class="link" href="Executor1.html" title="Executor requirements">Executor
        requirements</a>. The type <code class="computeroutput">Executor2</code> is <code class="computeroutput">associated_executor_t&lt;CompletionHandler,
        Executor1&gt;</code>. Let <code class="computeroutput">ex2</code> be a value of type <code class="computeroutput">Executor2</code>
        obtained by performing <code class="computeroutput">get_associated_executor(completion_handler,
        ex1)</code>.
      </p>
<h5>
<a name="asio.reference.asynchronous_operations.h9"></a>
        <span><a name="asio.reference.asynchronous_operations.outstanding_work"></a></span><a class="link" href="asynchronous_operations.html#asio.reference.asynchronous_operations.outstanding_work">Outstanding
        work</a>
      </h5>
<p>
        Until the asynchronous operation has completed, the asynchronous operation
        shall maintain:
      </p>
<p>
        &#8212; an object <code class="computeroutput">work1</code> of type <code class="computeroutput">executor_work_guard&lt;Executor1&gt;</code>,
        initialized as <code class="computeroutput">work1(ex1)</code>, and where <code class="computeroutput">work1.owns_work()
        == true</code>; and
      </p>
<p>
        &#8212; an object <code class="computeroutput">work2</code> of type <code class="computeroutput">executor_work_guard&lt;Executor2&gt;</code>,
        initialized as <code class="computeroutput">work2(ex2)</code>, and where <code class="computeroutput">work2.owns_work()
        == true</code>.
      </p>
<h5>
<a name="asio.reference.asynchronous_operations.h10"></a>
        <span><a name="asio.reference.asynchronous_operations.allocation_of_intermediate_storage"></a></span><a class="link" href="asynchronous_operations.html#asio.reference.asynchronous_operations.allocation_of_intermediate_storage">Allocation
        of intermediate storage</a>
      </h5>
<p>
        Asynchronous operations may allocate memory. [<span class="emphasis"><em>Note:</em></span>
        Such as a data structure to store copies of the <code class="computeroutput">completion_handler</code>
        object and the initiating function's arguments. &#8212;<span class="emphasis"><em>end note</em></span>]
      </p>
<p>
        Let <code class="computeroutput">Alloc1</code> be a type, satisfying the <a class="link" href="ProtoAllocator.html" title="Proto-allocator requirements"><code class="computeroutput">ProtoAllocator</code></a>
        requirements, that represents the asynchronous operation's default allocation
        strategy. [<span class="emphasis"><em>Note:</em></span> Typically <code class="computeroutput">std::allocator&lt;void&gt;</code>.
        &#8212;<span class="emphasis"><em>end note</em></span>] Let <code class="computeroutput">alloc1</code> be a value of type
        <code class="computeroutput">Alloc1</code>.
      </p>
<p>
        A completion handler object of type <code class="computeroutput">CompletionHandler</code> has an
        associated allocator object <code class="computeroutput">alloc2</code> of type <code class="computeroutput">Alloc2</code>
        satisfying the <a class="link" href="ProtoAllocator.html" title="Proto-allocator requirements"><code class="computeroutput">ProtoAllocator</code></a>
        requirements. The type <code class="computeroutput">Alloc2</code> is <code class="computeroutput">associated_allocator_t&lt;CompletionHandler,
        Alloc1&gt;</code>. Let <code class="computeroutput">alloc2</code> be a value of type <code class="computeroutput">Alloc2</code>
        obtained by performing <code class="computeroutput">get_associated_allocator(completion_handler,
        alloc1)</code>.
      </p>
<p>
        The asynchronous operations defined in this library:
      </p>
<p>
        &#8212; If required, allocate memory using only the completion handler's associated
        allocator.
      </p>
<p>
        &#8212; Prior to completion handler execution, deallocate any memory allocated using
        the completion handler's associated allocator.
      </p>
<p>
        [std_note The implementation may perform operating system or underlying API
        calls that perform memory allocations not using the associated allocator.
        Invocations of the allocator functions may not introduce data races (See
        C++Std [res.on.data.races]).]
      </p>
<h5>
<a name="asio.reference.asynchronous_operations.h11"></a>
        <span><a name="asio.reference.asynchronous_operations.execution_of_completion_handler_on_completion_of_asynchronous_operation"></a></span><a class="link" href="asynchronous_operations.html#asio.reference.asynchronous_operations.execution_of_completion_handler_on_completion_of_asynchronous_operation">Execution
        of completion handler on completion of asynchronous operation</a>
      </h5>
<p>
        Let <code class="computeroutput">Args...</code> be the argument types of the completion signature
        <code class="computeroutput">Signature</code> and let <code class="literal"><span class="emphasis"><em>N</em></span></code>
        be <code class="computeroutput">sizeof...(Args)</code>. Let <code class="literal"><span class="emphasis"><em>i</em></span></code>
        be in the range [<code class="computeroutput">0</code>,<code class="literal"><span class="emphasis"><em>N</em></span></code>).
        Let <code class="literal">T[sub <span class="emphasis"><em>i</em></span></code>] be the <code class="literal"><span class="emphasis"><em>i</em></span></code>th
        type in <code class="computeroutput">Args...</code> and let <code class="literal">t[sub <span class="emphasis"><em>i</em></span></code>]
        be the <code class="literal"><span class="emphasis"><em>i</em></span></code>th completion handler argument
        associated with <code class="literal">T[sub <span class="emphasis"><em>i</em></span></code>].
      </p>
<p>
        Let <code class="computeroutput">f</code> be a function object, callable as <code class="computeroutput">f()</code>, that
        invokes <code class="computeroutput">completion_handler</code> as if by <code class="literal">completion_handler(forward&lt;T[sub
        <span class="emphasis"><em>0</em></span></code>&gt;(t[sub <span class="emphasis"><em>0</em></span>]), ...,
        forward&lt;T[sub <span class="emphasis"><em>N-1</em></span>]&gt;(t[sub <span class="emphasis"><em>N-1</em></span>]))].
      </p>
<p>
        If an asynchonous operation completes immediately (that is, within the thread
        of execution calling the initiating function, and before the initiating function
        returns), the completion handler shall be submitted for execution as if by
        performing <code class="computeroutput">ex2.post(std::move(f), alloc2)</code>. Otherwise, the completion
        handler shall be submitted for execution as if by performing <code class="computeroutput">ex2.dispatch(std::move(f),
        alloc2)</code>.
      </p>
<h5>
<a name="asio.reference.asynchronous_operations.h12"></a>
        <span><a name="asio.reference.asynchronous_operations.completion_handlers_and_exceptions"></a></span><a class="link" href="asynchronous_operations.html#asio.reference.asynchronous_operations.completion_handlers_and_exceptions">Completion
        handlers and exceptions</a>
      </h5>
<p>
        Completion handlers are permitted to throw exceptions. The effect of any
        exception propagated from the execution of a completion handler is determined
        by the executor which is executing the completion handler.
      </p>
</div>
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
<td align="left"></td>
<td align="right"><div class="copyright-footer">Copyright &#169; 2003-2018 Christopher M. Kohlhoff<p>
        Distributed under the Boost Software License, Version 1.0. (See accompanying
        file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
      </p>
</div></td>
</tr></table>
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